Bull. Mater. Sci., Vl. 14, N. 2, April 1991, pp. 167-171.,E; Printed in India. A methd fr preparatin f a high-quality high temperature supercnducting ceramics L A KOTOK, L A KVICHKO, V T ZAGOSKIN, Yu G LITVINENKO and E T MOGHILKO Institute fr Single Crystals, Lenin a~.60, Kharkv 310141, USSR Abstract. An imprved methd fr ht pressing f high temperature supercnducting (HTSC) pwders prepared by cnventinal slid phase synthesis f the initial BaCO3, CuO and Ln20.~ allws ne t btain HTSC targets and magnetic shields pssessing a high degree f hmgeneity. By sputtering such targets HTSC films with critical current density f 3.3-10 ~ A/cm 2 have been depsited. Shields, prepared accrding t this methd, shw a shielding cefficient f l05 and tt,. f 79 (3e in the cnstant magnetic field and in the alternating magnetic field the amplitude is 90 Oe in the frequency range f 70 3000 Hz. Keywrds. Ht pressing; high temperature supercnducting ceramics; magnetic shields; slid phase synthesis. I. Intrductin In an attempt t wrk ut a methd fr preparatin f HTSC ceramics the physicchemical regularities f the frmatin prcess, the regin f temperature stability and the decmpsitin kinetics f the supercnducting phase in the cmpunds f the cmpsitin LnBazCu30%,, where Ln = La, Y, Sm, Eu, Gd, Dy, Er, Tm, H, Yb have been studied. Differential thermal analysis (DTA), thermgravimetry (TG), X-ray diffractin analysis including high temperature, dilatmetry, electrn micrscpy and electrn-prbe micranalysis have been used fr this study. Determinatin f regularities f phase frmatin has made it pssible t ptimize cnditins fr the synthesis and develp a methd fr preparatin f HTSC materials with a reprducible cmplex f the set prperties by a ceramic technique using raw materials with varius prehistries. 2. Results and discussin The scaled-up prductin technique fr materials with preset prperties is based n a well-knwn prcedure f slid-phase interactin f BaCO3, CuO and Ln2 03 initial cmpunds during heat treatment. The electrphysical prperties f HTSC cmpunds strngly depend n their phase cmpsitin defined by synthesis cnditins. We have studied the dependence f supercnducting prperties n the phase cmpsitin, xygen partial pressure and synthesis temperatures f LnBazCu30 v_7,- type cmpunds (where Ln = La, Y, Sm, Eu, Gd, Dy, Er, Tm, H, Yb). By means f thermgravimetry, dilatmetry, X-ray diffractin and electrn-prbe micranalysis it has been established that the 1 2 3 supercnducting phase frmed via the intermediate cmpunds LnzBaCuO 5 (I) and BaCuO2 {II) is stable in a narrw Tf... -Td... p temperature interval and is decmpsed int the same cmpunds I, II and CuO (IIIj. The values f T {i.e. the temperature f 1-2 3 cmpund frmatin) fr varius 167
168 L A Ktk et al rare-earth elements are in the range f 840-980 C. The values f decmpsitin temperature (Td... p) increase with increase f rare-earth element radius and xygen partial pressure (Seminzhenk et al 1988; Blank et al 1988). The duratin f the initial material heat treatment in the prcess f synthesis is limited by the interactin rate f barium carbnate with cpper xides and rare-earth elements at a preset temperature. T ptimize the use f pwders required fr preparatin f ceramics, the kinetics f 1-2-3 phase frmatin has been investigated by means f heat treatment f Y203, CuO and BaCO 3 stichimetric cmpund. The interactin degree f the cmpnents has been determined with respect t the residual carbnate in cntent in the samples which have undergne isthermal treatment f varying duratins. The kinetics f the prcess calculated with respect t the interactin degree is in gd agreement with the results f 1-2-3 phase cntent determinatin using X-ray phase analysis. At varius stages f the synthesis pressed pwder samples have been made, their phase cmpsitin and Tc being determined. The best quality samples have been btained when the 1-2-3 phase cntent is nt less than 95% (figure 1). The prductin technique cnsists f preparatin f the initial cmpnents, their mixing in stichimetric rati and heat treatment with a gradual temperature rising up t T. Heating is repeated several times, the tablets being grund and hmgenized befre every heating. The residue mainly cnsists f particles up t 5-6 #m. It has been fund (Antnva et al 1988) that the tw-stage prcess f ceramic sample preparatin including pressing at rm temperature and subsequent heating prevents essential densificatin f HTSC ceramics. Hwever, simultaneus pressure and heating effects prvide a cnsiderable increase f density and prmte the preparatin prcess. Synthesized HTSC pwder is briquetted in dies made f Cr-Ni ally and pressed at a temperature f 600-900 C and a pressure up t 0.6 GPa t btain ceramics. Heating, cling, pressure lading and unlading rates are chsen by experiments 100 t 3 ~60 20 1 1 1 I I I 0 2 4 6 8 10 12 n, cycles 12hr durtin eech Figure 1. 1-2-3 phase cntent (A) and transfrmatin degree (O) in pwder and tablets vs heat treatment time.
Preparatin f high quality H TSC ceramics 169 t prevent the samples frm heat cracking and t prvide the cmplete transitin f 1 2 3 tetragnal phase int rthrhmbic mdificatin (Zagskin et al 1988). By pressure and ht-pressing temperature variatins the ceramic density may be mdified ver a wide range, as shwn in figure 2. Special attentin shuld be paid t the hmgeneity f temperature distributin ver the samples, since the tablet density is inhmgeneus alng the sample diameter wing t the temperature gradient n the plane perpendicular t cmpressing pressure. This inhmgeneity may be eliminated by chsing prper heating cnditins fr the die and prper arrangement fr heating. The reductin f the temperature difference between the tablet centre and its side surface favurs preparatin f density-unifrm ceramics (figure 3). 6.0 8OO*C 5.6 ~5.2 4.8 4.4 4.0 Figure 2. pressing. l 300 500 700 MP YBa2Cu]O1-7, ceramics density dependence vs pressure and temperature f ht 5-6 I _ 1 I- I_ AT : 20"C r, 5-2 5.0 ~12 60 40 20 0 20 40 60 (rnm) Figure 3. Radial distributin f the material density f the ceramic HTSC target YBa2Cu307 r, dia68mm.(a) AT- 80 C, (b) AT= 20 C.
170 L A Ktk et al X - Oxygen index in YB z Cu 3 07-,, 0.3 0.2, I 0 I 1 I I 1 I I 1 I l 5 10 (ram) Figure 4. Oxygen cntent (index,c) in the material f the YBa2Cu3OT_ ~. target at varius distances frm its surface. (Q) befre heating. (A) after additinal heating. 5.6 5.4 I 6kber 2 k bar..- II..u 5,2 (.9 5.Q 4.8 I ] I 0 40 80 120 Time (h) 1 160 Figure 5. YBazCu307,., ceramics density vs pressure f ht pressing and the time f additinal heating. 100 1.6 ~96 0,-~ II -" ~, 1.2 ~ ~-u94 92 Figure 6. treatment. I I L ] I i 0.4 0 40 80 120 160 200 240 Time (h) Characteristics f the transitin int the supercnducting state vs time f heat
Preparatin ~[ h iyh quality H TSC ceramics 171 Additinal heating f ht-pressed ceramics at 850 940C allws eliminatin f xygen deficit n the surface (figure 41 t increase ceramic density (figure 5) and imprve its supercnducting prperties (figure 6). The latter effect is prbably cnnected with the cncentratin decrease f impurities lcated at grain bundaries and cmpensatin f xygen nn-stichimetry. On the basis f these studies the preparatin technique fr LnBa2Cu3OT_y (where Ln = La, Y, Yb, H, Er, Eu, Gd, Dy, Sm, Tm} targets has been develped. The targets may be made in the frm f disks (10 210mm in diameter and 2 20mm thick} r cylinders (with uter and inner diameters f 15 20 and 6 8 mm respectively and 10-30mm high). YBazCu30~-v films btained using these targets have the fllwing characteristics: ~/~,=90 92K, AT,.~1.5 2K, l,.~3.3x 10 /~/cm 2 at 77.4K. A technlgy fr manufacturing magnetic shields frm the YBazCu30 7_ v pwder by an isstatic cmpressin has als been elabrated. Density f the shield material varied frm 78 t 93",, with respect t the theretical value, depending n the particular cnditins f pressing. Size f mdel samples f shields is as fllws: uter diameter 15 20mm, inner diameter 6 12mm and well depth 40 60mm. The shielding cefficient, n lwering the temperature frm 93 t 90 K, varied frm 1 t nt less than 2 x 104 fr the lngitudinal external magnetic field. In the transverse magnetic field the shielding cefficient in the frequency range f 70-3000 Hz was nt less than 105 up t the field H,l = 79 Oe. Unfrtunately the sensitivity f the measuring path prevented estima'tin f shielding cefficient with higher accuracy. The absence f frequency dependence f the shielding cefficient reveals that the true ne is 1 2 rders higher. 3. Cnclusins A methd has been wrked ut fr the preparatin f high-quality HTSC ceramics with 1 2 3 cmpsitin and which pssesses high supercnducting characteristics, structural and phase unifrmity. This methd enables manufacture f varius ceramic articles, including targets fr magnetrn and laser sputtering, their size reaching 210mm. Films btained frm these targets have Ic f 3-3 106A/cm2(77 K) with T = 90-92 K and ATc = 1.5-2 K. Shields prepared accrding t this methd have a shielding cefficient f 105 with Hc = 79 Oe in the cnstant magnetic field and in the alternating magnetic field the amplitude is 90 Oe in the frequency range f 70-3000 Hz. References Antnva V V et a11988 Study f the cnditins fr the preparatin, cmpsitin and prperties f dense supercnducting ceramics Y-Ba-Cu-O, Preprint Inst. Metal-lfi~iki Akad. Nauk UkrSSR. Kiev. N IMF-13-88, p. 5 Blank A B, Zagskin V T, Kvichk L A, Ktk LA, Litvinenk Yu G and Seminzhenk V P 1988 Physica 153-155 pp. 1663 Seminzhenk V Pet al 1988 Dkl. Akad. Nauk. Ukr.SSR, ser. A, p, 50 Zagskin V T, Kuznetsv V P, Lakin E E, Litvinenk Yu G, Puzikv V M, Mghilk E T and Seminzhenk V P 1988 11 Eurpean Crystall,qr. Meetin(4, Pr,qramme chan~4es and late abstracts, Austria, p. 9